The determinants of respirator tolerance appear to be multifactorial. This study will utilize a human panel method for the assessment of the integrated effects of respirators and will examine the effects of several types of respirator loads to gain understanding of the mechanisms by which workers tolerate respirator use. Panels of human subjects will participate in a series of studies. Outcome measures will include physiologic variables (e.g., respiratory timing, ventilation, ventilatory work), subjective tolerance (measured by visual analog scales), and work performance ability. Psychophysiologic sensitivity to loads (both resistive and pressure bias) will be measured as well. Studies will be performed in a laboratory setting, in a prescribed field course (during which the subject may self pace), and at worksites. Respirator surrogate loads and actual respirators will be employed, and several types of exercise will be employed. The inter-relationships between and among the predictor (e.g., respirator load, exertion type) and outcome measures will be assessed to define mechanisms of respirator effects and to develop accurate and reasonably simple means for human panels to test respirators. The experimental design permits complex analyses by having each subject participate in a series of studies and by repeating certain experimental conditions over time. Subprojects will address additional specific problems. The effects of pressure biasing (as occurs with many SCBA's) upon physiologic, subjective, work performance, and lung volume measures will be tested. A group of self-identified workers who are particularly intolerant of respirators will participate. The use of respirators in simulated "emergency use" situations will be assessed in comparison to more routine use.